Embodiment No. 1
A number of prior-art processes have been described for the preparation of oxalate esters by the oxidative carbonylation of alcohols with carbon monoxide. These prior-art processes generally involve the use of oxygen or oxygen-containing gases, noble metal salt catalysts, and metal oxidant salts as well as dehydrating agents or azeotropic distillations to eliminate the presence of water.
The present invention describes an improved process for the oxidative carbonylation of alcohols in which an alcohol is reacted with carbon monoxide in the presence of a noble metal salt catalyst with or without the corresponding metal oxidant salt and with a substituted or unsubstituted 2,5-cyclohexadiene-1,4-dione as oxidant at suitable temperatures and pressures. The 1,4-dihydroxybenzene formed from the above reaction may be reoxidized by the process described in Embodiment No. 3, infra, to provide a cyclic process or may themselves be purified for sale as a valuable industrial chemical.
U.S. Pat. No. 3,393,136 describes a process in which carbon monoxide is oxidized in the presence of a platinum group metal catalyst, a ferric or cupric metal oxidant salt, and a water scavenger such as orthoformate ester. The salts are returned to their oxidized states through the introduction of oxygen or oxygen-containing gases or by application of a direct current electrical potential to the reaction zone. When oxygen is employed, explosive mixtures of organic vapors in the gas phase must be avoided.
A West German Pat. No. 2,213,435 describes a method for the synthesis of oxalic acid and oxalate esters. Water or alcohol is reacted with carbon monoxide and oxygen in the presence of a platinum group metal salt, the salt of a metal more electropositive than platinum, and an alkali metal salt. A disadvantage of this reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect the reaction.
A series of patents (U.S. Pat. No. 3,994,960; German Offen. No. 2,514,685; Japan Kokai Nos. 77 31,015, 76 29,424, 76 95,031 and 76 105,008) have been issued to T. Yamasaki and coworkers, disclosing a process for the preparation of dialkyl oxalates utilizing carbon monoxide and oxygen. Carbon monoxide, alcohol, and small increments of oxygen are reacted in the presence of a platinum group metal salt, a salt of copper or iron, and an accelerator composed of one or more components. The use of promoters is said to decrease the necessity for maintaining anhydrous conditions. With the use of oxygen, however, explosive mixtures may still be formed.
L. R. Zehner has also been issued a series of patents (U.S. Pat. Nos. 4,005,128-131; 4,041,067-8; 4,065,490; 4,069,388; 3,992,436; German Offen. Nos. 2,721,734 and 2,808,574) describing similar processes. These processes involve the reaction of carbon monoxide, alcohols, or other alcohol sources and oxygen or oxygen-containing gases in the presence of a platinum group metal salt or complex, a metal oxidant salt, and an amine base or the salt of an amine base. Again, the use of oxygen permits the possible formation of explosive mixtures.
D. M. Fenton and P. J. Steinwand (J. Org. Chem. 39 (5), 701-4, 1974) have described the synthesis of oxalate esters by the reaction of carbon monoxide and alcohol in the presence of a palladium (II) chloride-copper (II) chloride redox couple, oxygen, and dehydrating agents. The use of benzoquinone as an oxidant and palladium (II) chloride catalyst with or without dehydrating agents is shown. In the absence of dehydrating agents and at low carbon monoxide pressures and long reaction times, only low yields of oxalate (2.4 percent) and carbonate (2.8 percent) were obtained.
U.S. Pat. No. 4,005,130 discloses a process for the production of oxalate esters by the oxidative carbonylation of alcohols with carbon monoxide in the presence of a catalytic amount of copper, nickel, cadmium, cobalt, or zinc metal salt catalyst and a substituted or unsubstituted 2,5-cyclohexadiene-1,4-dione. Yields of oxalate esters approach 50 percent while oxalate to carbonate ratios of 2 to 4 were common. Yields were based on gas-liquid phase chromatography and do not reflect isolated esters.
The oxalate products of the present invention have many important commercial applications including their use as solvents for cellulose ether or ester resins, in the preparation of pharmaceuticals and glycols, and as intermediates in the production of dyes, agricultural products, and a number of potential slow-release fertilizers, a known example of which is oxamide.
Advantages of the present invention over the prior-art processes are: (1) the high conversion and selectivity toward oxalate esters over the competing carbonate esters, (2) the increased rate of reaction thus reducing the time necessary to effect the reaction, (3) elimination of the hazardous operational conditions by avoiding hazardous mixtures of oxygen and carbon monoxide, (4) avoiding the necessity of employing dehydrating agents to remove water, and (5) the recovery of the 1,4-dihydroxybenzene formed in the reaction in nearly quantitative yield for its subsequent regeneration or purification for sale as a valuable industrial chemical.